In a belt-type continuously variable transmission in which an endless belt is wound around a drive pulley provided on an input shaft and a driven pulley provided on an output shaft, the groove width of the pulley is changed by controlling the axial thrust with which the pulley compresses the endless belt, thus increasing/decreasing the gear ratio and preventing the occurrence of slip between the pulley and the endless belt. When the axial thrust of the pulley is controlled so as to prevent the occurrence of slip between the pulley and the endless belt, if the coefficient of friction between the pulley and the endless belt is large, it is difficult for slip to occur, but the amount of heat generated increases to thus possibly degrade efficiency, whereas if it is small, slip easily occurs, and it is therefore necessary to calculate a necessary axial thrust while taking into consideration the coefficient of friction.
Since the coefficient of friction between the pulley and the endless belt changes according to the gear ratio of the belt-type continuously variable transmission, a coefficient of friction map in which a default value (initial value set for a brand new car) of the coefficient of friction is prestored is provided for each gear ratio, and the axial thrust of the pulley is controlled by reading out the default value of the coefficient of friction from the coefficient of friction map according to a given gear ratio. However, when the properties of the contact faces of the pulley and the endless belt change due to wear, etc. caused by change over time, the coefficient of friction therebetween gradually changes, the actual coefficient of friction deviates from the default value of the coefficient of friction stored in the coefficient of friction map, and it is therefore necessary to correct the default value of the coefficient of friction based on the actual coefficient of friction calculated from running conditions of the belt-type continuously variable transmission and use it for control of the axial thrust of the pulley.
Patent Document 1 below describes an arrangement in which the actual coefficient of friction between the pulley and the endless belt is calculated for each gear ratio, the default value of the coefficient of friction stored in the coefficient of friction map for the corresponding gear ratio is corrected using the calculated coefficient of friction, and the axial thrust of the pulley is controlled using the corrected coefficient of friction.